Errors in time-difference-of-arrival (TDOA) and frequency-difference-of-arrival (FDOA) measurements used to perform geolocation of non-cooperative emitters (i.e., signals of unknown format or content, transmitted from an unknown location) can be reduced by use of a reference beacon that transmits a signal from a known position. By performing a reverse geolocation using signals transmitted from a reference beacon with a known position and velocity, it is possible to estimate bias errors in the TDOA/FDOA measurements taken by the collection system. The estimated bias errors can then be subtracted from measurements taken of non-cooperative emitter signals to reduce the bias error in those measurements. Such correction is typically performed by a reference correction processing system that is part of or operates in conjunction with the geolocation system.
In order to correctly compute the bias errors using signal data collected from a reference beacon transmitter, it is necessary to know the position (and/or velocity) of the transmitter at the time of transmission.
A reference beacon signal need not contain any information in order to provide useable bias corrections. A random or pseudorandom waveform with good correlation properties can be used as a reference beacon signal. In fact, it is generally simpler to generate a signal containing a pseudorandom waveform, than one which contains information.
Currently deployed reference beacon systems typically use stationary reference beacon transmitters whose locations are constant and known to geolocation processing systems, which compute the TDOA/FDOA bias error corrections based on the reference beacon signals and the locations of the transmitters. The beacon signals transmitted by these transmitters typically consist of pseudorandom waveforms, which contain no data.
The Boeing Company is currently developing a reference beacon system that can be placed on a moving platform (e.g., an aircraft). In such an arrangement, the position of the reference beacon transmitter is provided to the geolocation processing system over a network connection. This approach, however, requires the geolocation processing system to associate sets of position data received over the network from the reference beacon transmitter with sets of RF signal data independently received from one or more collection platforms. This association is accomplished by having the geolocation processing system command the reference beacon to transmit at a known time. In the aforementioned system, the reference beacon transmits its position data over the network at the time it starts transmission of the reference beacon signal. As with other conventional approaches, the reference beacon signals consist of pseudorandom waveforms.